Wind turbines are nowadays considered as a “green” source of energy and, in fact, belong to one of the most developed renewable technologies. Most wind turbines are horizontal-axis wind turbines having a rotor provided with three blades connected to a main rotor shaft and an electrical generator mounted within a nacelle at the top of a tower. Many conventional types of wind turbines comprise a gearbox that converts the relatively slow rotation of the wind turbine rotor into a faster rotation to drive the electric generator rotor. Electric generators and electric machines in general, exhibit poor efficiency when driven at very low speed. Moreover, slow electric machines have to exhibit higher torque in order to compensate for the reduced rotational speed of the machine rotor, since machine power output is always yield by torque times the rotational speed. Gearboxes reduce the efficiency of the conversion of wind energy into mechanical energy used for driving the electric generator rotor and are moreover rather expensive, need intensive and scheduled maintenance and are open to wear-out and thereto related failures. Direct-drive generators based on conventional electric machines technologies do not comprise gearboxes and thus lack the disadvantages thereof, but are intrinsically heavier and larger. This is remarkably the case of slow electric generators for wind turbines whose nacelles, equipped with such generators, are more difficult to lift, to place and to balance on towers so that they are rather unsuitable for high-power producing generators as wind turbines.
To overcome the drawbacks of conventional direct-drive generators, direct-drive synchronous generators comprising superconducting field windings providing an increased torque density for use in wind turbines have been described.
Superconductivity is inherent to certain materials generally at very low temperatures, leading to zero DC electrical resistance and the exclusion of the interior magnetic field (at given temperature and current density, there is a maximum magnetic field the superconductor can expel; as the field exceeds this value the superconductor becomes a normal conductor and exhibit ohmic resistance). Superconductivity occurs in a wide variety of materials, including simple elements like tin and aluminum, but also in other materials such as various metallic alloys and some heavily-doped semiconductors, in some lanthanum- and yttrium-based cuprate perovskite materials, ceramic materials consisting of thallium, mercury, copper, barium, calcium and oxygen, lanthanum oxygen fluorine iron arsenide. An especially interesting superconductor from a practical point of view is magnesium diboride, a conventional superconductor, that is relatively easy to synthesize and manufacture in long wires and which has a critical transition temperature of 39 K in bulk or powder form.
Usually, copper field windings of a conventional synchronous machine are found buried in the rotor slots or wound around rotor salient iron poles, while armature windings are hosted within slots and separated by iron teeth in the stator. These iron elements strengthen and efficiently guide the magnetic flux across the machine air-gap, that is, the radial separation existing between the rotor surface or rotor poles and the stator teeth. Superconducting field windings can carry much more current (Ampere·turns) than conventional ones, thus producing higher magnetic fields. Therefore, some iron elements can be dispensed. In superconducting machines, the magnetic circuit of the electric machine comprises two iron annularly shaped elements, the rotor back-iron and the stator back-yoke. To establish a high flux density in the large air-gap, a very high magnetomotive force is needed. With a configuration as such, with the superconducting field coils located above the annular rotor back-iron, the highest magnetic fields are usually found right on the superconductors. Most recent prototypes of superconducting machines implement High Temperature Superconductors (HTS) with a transition temperature above 77K (boiling point of Nitrogen) such as YBCO and BSSCO. These superconductors can take high fields (2.5-3.5 T) at a reasonably low temperature (20-40K). However, this class of superconductors is very expensive as BSCCO has silver matrix and YBCO is very difficult to manufacture in long pieces such as those required for coils, and the production requires time-consuming processes and complicated technologies. As a consequence, electric machines implementing HTS are too expensive.
Other materials, such as NbTi (Tc 9K) or Ni3Sn (Tc 18K), have been considered in the past for this application. However the performance of these superconductors is interesting at temperatures of 4.2K (boiling point of Helium) or below. Plants for liquifying helium are also very expensive. Moreover, the lower the temperature, the more unstable is the cryogenic system. These are serious disadvantages of LTS machines.
Magnesium diboride offers an interesting compromise: On the one hand, it is much cheaper and is operated at temperatures in the order of 15-25K. On the other hand, to carry high currents at these temperatures, existing wires based on magnesium diboride cannot take very high magnetic fields, usually well below 2 T. Therefore to achieve adequate performance, the magnetic circuit of superconducting synchronous electric machines based on magnesium diboride wires and cheap materials with similar characteristics should comprise salient iron poles protruding from the rotor back iron. The iron poles divert the magnetic flux from the superconducting coils and reduce the circuit reluctance.
Usually, superconductors are kept at the proper operating cryogenic temperature in special vessels called cryostats. Known prototypes of superconducting machines often exhibit a “cold rotor” design, where a cryostat encloses                the superconducting coils        the rotor back-iron        the rotor frame.        
The “cold rotor” design as the one disclosed in WO-A 2007/033858 is not suitable for salient poles rotor, because the cryostat external jackets above the poles would require extra space, thus making the air-gap longer and having a negative effect in the magnetic field distribution. This feature would reduce the efficiency of the magnetic circuit.
In the “wet cold” system as the one disclosed in patent EP-A-1959548, the super conductive part is immersed in a coolant, in gas or liquid state, and is isolated from the outside in the cryostat. Heat generated by external or internal sources is exchanged with the coolant and is extracted outside the cryostat in order to keep the machine under the proper operation temperature. In case of liquid coolant, the cooling procedure may involve a phase change to the gas state. Both gas He and liquid He (boiling temperature 4.2K) have been widely used as coolant. As He is a very expensive element, several closed-cycle systems have been developed to recover these expensive gas, and, even, liquefied it again. As well, some machine prototypes have been developed with cooling systems based on liquid Ne (boiling temperature 27K) and liquid nitrogen (boiling temperature 77K), depending on the SC material. All of them need a continuous re-filling of the cryostat or expensive recovery systems based on liquid plants.